1. Field of the Invention
The present invention relates to an optical sheet, and more particularly, to an optical sheet and a backlight unit having the same.
2. Discussion of the Related Art
Liquid crystal display devices (LCDs) display images by controlling pixels arranged in a matrix configuration to adjust the displacements of liquid crystal (LC) molecules and thus adjust light transmittance. Because the LCDs do not generate light by themselves, the LCDs receive light from an external light source, e.g., a backlight unit.
FIG. 1 is a view of a backlight unit of the related art LCD.
Referring to FIG. 1, the LCD includes: a lamp 10 disposed at a side of the LCD to generate light; a lamp housing 14 enclosing the lamp 10; a light guide plate 12 for changing light incident from a lateral direction from the lamp 10 into a surface light source; a reflector 16 disposed below the light guide plate 12 to reflect the light upward; a diffuser sheet 18 disposed above the light guide plate 12 to diffuse the light; a prism sheet 20 for controlling the direction of the light that has passed through the diffuser sheet 18; and a protection film 24 formed on the prism sheet 20.
An LC panel (not shown) is located on the backlight unit to realize an image by controlling the transmittance of the light emitted from the backlight unit.
The related art backlight unit illustrated in FIG. 1 is an edge-type backlight unit where the lamp 10 is disposed at the side of the backlight unit. In the case of a direct-type backlight unit where a plurality of lamps are arranged on the same plane, the light guide plate 12 may be omitted.
Cold cathode fluorescence lamps (CCFLS) typically are used for the lamp 10. Light generated by the lamp 10 is incident to the light guide plate 12 through an incident plane located on a lateral side of the light guide plate 12. A reflection surface on the inner surface of the lamp housing 14 reflects light from the lamp 10 toward the incident plane of the light guide plate 12.
The light guide plate 12 includes a light-emitting plane approximately parallel to an inclined bottom plane of the light guide plate 12. Also, the light guide plate is manufactured such that the incident plane is substantially perpendicular to the inclined bottom plane. The reflector 16 is installed on the bottom plane of the light guide plate 12 facing the bottom plane. The light guide plate 12 allows light incident from the lamp 10 to reach a place distant from the lamp 10. The light guide plate 12 is formed of polymethylmethacrylate (PMMA) with high strength that is not easily transformed or broken and having excellent transmittance.
The reflector 16 reduces light loss by reflecting light incident to below the light guide plate 12 in an upward direction. When light from the lamp 10 is incident to the light guide plate 12, the light is reflected with a predetermined inclination angle at the bottom-inclined plane to uniformly progress toward the light emitting plane. The light emitted from the light guide plate 12 by way of the light emitting plane, is diffused to the entire region of the LC panel by the diffuser sheet 18.
Light incident to the LC panel has a high light efficiency when the light is perpendicular to the surface of the LC panel. For that purpose, the prism sheet 20 is provided to allow the direction of the light from the light guide plate 12 to be substantially perpendicular to the LC panel.
The prism sheet 20 includes a plurality of prism rods each having a crest and a valley. The prism sheet 20 changes the diffused light emitted from the diffuser sheet 18 into light substantially perpendicular to the LC panel. The protection film 24 is disposed on the prism sheet 20 to protect the surface of the prism sheet 20 and to diffuse the light to achieve uniform distribution of light.
FIGS. 2A and 2B are a sectional view and a perspective view of the prism sheet illustrated in FIG. 1.
Referring to FIGS. 2A and 2B, the related art prism sheet 20 includes: a substrate 21 to which light diffused by the light guide plate and the diffuser sheet is initially incident; and a isosceles triangular prism-shaped protrusion 22 having a predetermined vertical angle α, for allowing the diffused light to progress along a constant optical path. The protrusion 22 is arranged in a stripe shape on the substrate 21.
When a vertex 23 of the protrusion 220 receives light and faces the LC panel, the diffused light incident from the substrate 21 is refracted and gathered. However, when the light incident to an inclined plane 24 of the protrusion 22 is not emitted due to a total internal reflection, such light is lost, and does not contribute to brightness at the front of the LC panel.
To overcome this problem, the arrangement of the prism sheet may be changed such that the vertex faces the light guide plate so as to use the vertex 23 for an incident plane. In that case, the front brightness may be improved but a viewing angle becomes narrow, that is not appropriate for flat display devices that require a wide viewing angle such as a television for home use.
Also, two prism sheets may be used such that the arrangement directions of the protrusions 220 overlap and intersect each other. In that case, the number of parts and manufacturing costs of the LCD increase.